The only time I ever experienced it was when I changed my method of entering the lucid dreamstate. I learned that in order to avoid it, you must learn to let your automatic body functions go one way, and the mind its own. So, just let your body go into normal sleep. Do not change how your body goes to sleep.

You see, if you follow your body into sleep, you will not lucid dream. If your body follows you into the lucid state, there will be paralysis. So, there are two paths that do not give the desired result. Lucid Dreaming aids in increased awareness, increased perception. This division in mind is actually basic perception, distinction.

It is just an error in execution. It is bound to happen to anyone who has not acquired the right habits. It is often triggered by fear. When you have learned the right habits, you will not need any aid at all. As I learned, you will eventually close your eyes and simply start walking. You will end up where you want to go.

As the other said, fear or even just expectation of the sleep paralyses state can cause a person to have it eg thinking that this happens for a LD to be able to occur can cause it. So this can be a problem of the more you want it to stop (so think about it more), the more you may get it. Mastering any fears you have around it so you do not mind this state anymore is one way in which it may stop.

You may also need to break whatever habits you have which lead to this state as the body gets into habits of doing certain things eg if you are in the habit that laying on your back will cause SP, it probably will keep doing so. (trying to LD in a different position may help break the habit of going into it rather then straight into a dream where you can move around in).

Avoid unpredictable sleep pattern and get a lot of sleep. Individuals who are sleepless or who have irregular sleep examples (like movement specialists) can have irritated REM rest. Since sleep loss of motion is a "breakdown" of REM, irritated REM rest likely makes individuals powerless against sleep loss of motion.